Date: Fri, 19 Sep 1997 09:59:39 +0930 From: Mike Smith <mike@smith.net.au> To: joerg_wunsch@uriah.heep.sax.de (Joerg Wunsch) Cc: hackers@freebsd.org Subject: Re: INB question Message-ID: <199709190029.JAA02973@word.smith.net.au> In-Reply-To: Your message of "Thu, 18 Sep 1997 22:18:39 %2B0200." <19970918221839.VL10449@uriah.heep.sax.de>
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> As Mike Smith wrote: > > > > The ISA specification explicitly requires bus pullup resistors. It may > > be unwise to depend on reading 0xff back-to-back with a previous read/ > > write operation, ... > > That's why i wrote ``unspecified, with a tendency to 0xff''. The implication (as an english speaker) from your claim was "unspecified but sometimes 0xff". It would be civilised to qualify the "tendency" under the circumstances. > > but the reader is welcome to calculate the RC time constant for a > > transmission line with a few pF of capacitance and a 10K (or less) > > pullup. > > j@uriah 132% perl -e 'print 50e-12 * 10e3; print "\n"' > 5e-07 > > I think 50 pF is rather an understatement. 0.5 µs doesn't seem to be > terribly short, but IIRC, ISA inb's are artificially deferred by 1.25 > µs, so chances are good to actually see 0xff. I wouldn't rely on it > for a back-to-back read, however. 50pf is a little on the low side, granted. I don't have my copy of Solari handy to check, but a quick eyeball of the boards to hand indicates that something lower like 4k7 is common practice, so 500ns drift-up time is not unreasonable. OBTW, see my trailing comment wrt. transfer rates; if ISA read cycles are deferred by 1.25us, how do I manage 1.3MW/sec from a user-space process? (This is with a P166 on an HX board; nothing special.) mike
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